RJ11 – oddities between headset and handset connectors?

Well at least in my case, this is what it looks like.
I got a 3.5mm -> RJ11 adapter to connect a normal headset to my landline phone instead of the receiver and at first nothing worked – I was about to give up as I thought “nothing to lose – just switch the two 3.5mm cables for speaker and microphone”(even though they were connected correctly according to their symbols/colours) – and hey – it works, even though strictly speaking it shouldn’t